1. Field of the Invention
This disclosure relates to the field of electrical power conversion, specifically to power inverters and methods to reduce harmonics in a direct current (DC) to alternating current (AC) power inverter by providing a reference signal having an irregular period.
2. Background of the Invention
Power inverters are a commonly used device for providing alternating current (AC) power to devices which utilize AC power input when traditional AC power grids or other infrastructure are not available. In particular, a power inverter is designed to take in direct current (DC) power and convert it to a form which approximates traditional AC power.
The DC power source supplying an inverter may be provided in any format and from any source. In many common applications of power inverters, the DC power source is an alternative electrical power generator such as a fuel cell, solar panel, or wind turbine. In other applications, the DC power is obtained from an internal combustion or similar engine that mechanically drives a permanent magnet alternator. These systems are commonly used to provide for readily available AC power in emergencies or in locations lacking reliable power infrastructure. DC power may also be obtained from rectified AC power. DC to AC power converters are also commonly used on board vehicles where DC power may be readily available from the vehicle's engine compartment but AC power is generally not easily obtained.
AC power is often a more useful source of energy than DC power for a variety of reasons. The most common is often that AC power allows the use of standard off the shelf equipment designed to operate off a power grid providing AC power. DC powered versions of these appliances are often bulkier and less efficient, incurring higher energy losses. Further, certain types of appliances require AC power in operation as only an AC voltage provides the required operational characteristics. While the general concept of AC power is fairly common, AC power may be provided at a variety of different voltages and frequencies in different parts of the world, and with specialized applications such as for use on aircraft.
In remote areas where power infrastructure may not exist, or may have been disabled due to military action or disaster, it is generally necessary to provide generators to produce power to operate electrical devices. In many of these situations, AC power is necessary because devices to be operated were previously used with an AC power grid which is no longer operating. Further, as the military, and other groups, become increasingly reliant on electronic technology and other electrical devices to operate efficiently, the need for AC power generators has increased dramatically in order to allow them to carry out necessary tasks.
While the ability to produce AC power from DC power through the use of an inverter is a relatively simple basic technology, many modern AC powered devices are sensitive to the “cleanliness” of the power provided to them and to the electromagnetic interference (EMI) present in the environment in which they operate. Advanced electronics including, but not limited to, communication devices, computers, and sensing equipment may have their performance impaired by electrical noise and EMI if their AC power source is not of sufficiently high quality. This noise can be distracting to the operator and can reduce the operating effectiveness of the device. In many modern applications, this is unacceptable as it can cause problems in using devices essential for military, disaster recovery, medical, or other critical applications.
The problem of noise in a DC to AC power converter often arises because the power inverter, while producing an AC power output, does not actually produce a “true” AC voltage signal. A “true” AC power source such as a power utility grid generator transfers power in a form closely resembling a sine wave. Therefore, the AC voltage signal is considered an alternating signal with a predetermined frequency. DC power, however, generally is transferred at a constant voltage signal. Therefore, when starting with a DC power source, the power inverter will generally provide some form of square wave output signal instead of a sine wave.
In creating from a DC power source AC power that is similar to that obtained from a true sine wave, one technology which is particularly useful is pulse-width modulation (PWM) which is also called pulse-duration modulation (PDM). The PWM circuit provides a set of electronic switches, typically insulated gate bipolar transistors (IGBTs). A control logic, such as, but not limited to, processor or electronic system is used to turn the switches on and off in a well orchestrated manner to produce a sequence of pulses of a fixed positive and negative magnitude and a varying duration whose root-mean-square (RMS) value approximates that of a sine wave.
While this system is effective at producing relatively good AC power at a desired frequency, there is a significant presence of undesirable signals in the resultant AC voltage signal from this methodology due to the switching action generating other constructive interactions. The frequencies of these signals are generally integer multiples of the frequency of the switching signal. For instance, if the desired AC signal is at 400 Hz, as is commonly used in many aircraft applications, and the reference frequency is 10 kHz, then the switching frequency is 20 kHz and undesirable signals are usually seen at 20 kHz, 40 kHz, 60 kHz and other integer multiples. The first harmonic (20 kHz in this case) is often the most problematic due to its relatively large magnitude. The interference can range from a simple nuisance noise such as a hum or static on a communication channel, to actually causing overheating or other dangerous situations in the device and therefore control of harmonics is quite important.
Traditionally, harmonics have been dealt with by attempting to remove or attenuate them from the AC voltage signal prior to power use by an electrical device, instead of trying to eliminate their production. IEEE standard 519-1992 for example, the entire disclosure of which is herein incorporated by reference, provides for suggested practices for dealing with harmonics. The standard generally recommends filtering of power lines prior to power use to attenuate problematic harmonics, and to design switching angles to eliminate some harmonics. Further, internally wiring structures to handle harmonics is also recommended.
While these standards are a logical way to attempt to deal with harmonics, they all suffer from the fundamental flaw that they do not eliminate the generation of the harmonics, instead they simply try to attenuate them from the power line after generation. These processes, therefore, require the use of altered construction methods of AC powered devices and additional components to provide for filtering. It would be desirable to instead eliminate or minimize the generation of harmonics in the beginning as fewer downstream components and modifications would be needed.